In my above-cited copending patent application I describe a vise having a stationary jaw, a movable jaw displaceable longitudinally toward and away from the stationary jaw, an outer hollow high-speed spindle threaded in a nut on the fixed jaw and formed with a radially open entrainment recess, and an inner low-speed spindle threaded in the outer spindle. A piston-and-cylinder unit on the movable jaw has primary and secondary pistons respectively engaged by the outer and inner spindles. A hand crank is rotationally fixed on the inner spindle and an entrainment pin is displaceable transversely in the inner spindle between an entrainment position engaged in the recess and rotationally coupling the inner and outer spindles to each other and a freeing position clear of the recess and permitting relative rotation of the inner and outer spindles. This entrainment pin is formed with a longitudinally directed camming surface. A switching piece displaceable on the inner spindle between a high-speed position and a low-speed position can operate a bolt displaceable along the inner spindle and engageable with the entrainment pin and with the switching piece for pressing the operating bolt into the camming surface in the low-speed position of the switching piece and thereby pull the entrainment pin out of the recess to decouple the spindles and in the high-speed position frees the operating bolt from the camming surface and couples the spindles to each other.
The main problem with such a clamping device is that it is capable of exerting so much pressure on the workpiece in the low-speed position that it can crush the workpiece. The high mechanical advantage in the low-speed high-force position gives no feedback to the user to let him or her know that too much force is being exerted. Although it is possible to determine how much pressure is exerted by gauging how far in the inner spindle has gone, this is not a usable method in practice.
In German patent document No. 2,710,624 a device for limiting pressure comprises a rim formed on the actuating spindle that is engaged by a sleeve on the high-speed spindle. The sleeve is formed internally with grooves of various lengths into which a bolt carried on the actuating spindle can engage. The different-length grooves limit the stroke of the actuating spindle and thereby also set a limit to the pressure that can be exerted. This arrangement is not readily adaptable the system of the parent application. Since the spindle is provided with a transverse switching piece there are problems in making the adaptation. The result would be an excessively long high-speed spindle set up to make the stop sleeve accessible.